The present invention relates to recording and transmission of digital data, and more particularly, to methods of recording machine-readable media to inhibit unauthorized copying.
Digital information is often recorded on various machine-readable media such as optical discs for mass distribution. For example, computer software and audio files (such as music on compact disc (CD)), and video files (such as movies on digital video disc (DVD)) are commonly distributed on physical discs. Compact discs and DVD""s are subject to formatting standards for digitally recorded data, software, images, and audio. Herein after, the phrases xe2x80x9cdigital informationxe2x80x9d or xe2x80x9cdigital dataxe2x80x9d are intended to include, without limitations, software, audio, video, and other digitized information. Further, the word xe2x80x9cdiscxe2x80x9d includes, without limitation, optical and magnetic data storage disc devices (such as CD""s and DVD""s) and the phrase xe2x80x9cmachine-readable mediaxe2x80x9d includes without limitation magnetic tapes, solid state memory, and similar machine-readable data storage devices.
Stamped discs, such as CD""s and DVD discs, typically store information as a spiral track of embossed pits, or embossed marks. For example, a mark represents one of two binary digits, say a xe2x80x9c1,xe2x80x9d and the unmarked area, or xe2x80x9cspace,xe2x80x9d represents the other binary digit, say a xe2x80x9c0.xe2x80x9d The depth of these embossed marks is typically equal to or less than one quarter the wavelength of light used for reading the disc. This depth is carefully selected to provide near-maximum variation in intensity between reading of embossed marks and reading of spaces between the embossed marks while also providing a reliable tracking error signal. This design allows for near-maximal signal to noise ratio during the read operation.
Such discs do not contain the digital data in its original form. In fact, digital data are rarely recorded in its original digital form. Instead, high capacity digital recording typically involves numerous tradeoffs of various constraints and requirements, resulting in the original digital data being encoded into bit patterns that satisfy these constraints. A first constraint deals with a tradeoff between recording density and error rate. The need for a sufficiently small error rate imposes a requirement for additional information such as error correction code (ECC) to be added to the digital data for error detection and correction.
A second constraint deals with the highest permissible transition frequency, where a transition is a change from one media state such as an embossed mark to another media state such as an unmarked area. In magnetic data recording, a related limitation is commonly called intersymbol interference. Typically, in reading a recording medium such as a magnetic or optical recording medium, the signal produced by each transition of state has a distorting effect on the signal produced by neighboring transitions. This distortion imposes a maximum on the number of consecutive transitions that can be reliably read at a specified minimum transition spacing. In any recording medium, there is also a maximum allowable spatial frequency at which some physical phenomenon can switch states during recording or stamping. In magnetic recording media, the physical phenomenon is the direction of magnetic alignment of metallic particles. In optical stamped media, the physical phenomenon is the height difference between the unmarked surface and the pits (or bumps) of the medium. In recordable optical phase change media, the physical phenomenon is the crystaline phase of the recording medium, the two crystalline phases having different refractive indices.
A third typical constraint is self-clocking. For serial binary data, a clock signal for decoding the data often must be extracted from the timing of the transitions of a read signal (reversal of voltage or current, change of frequency or phase, change of light intensity, etc.). There must be an adequate frequency of transitions to keep the clock signal synchronized. Serial binary data are often physically in a format called Non Return to Zero Inverted (NRZI). In NRZI format, the waveform is at one state until a binary one occurs, at which time the waveform switches to an opposite state. The maximum transition rate, or intersymbol interference limitations discussed above, imposes a minimum on the amount of time that can pass between transitions. The requirement for self clocking imposes a maximum on the amount of time that can pass with no transition. A code that satisfies the maximum transition rate constraint, the self-clocking constraint, and the NRZI format requirements is commonly called a Run Length Limited (RLL) code. In a RLL code, the number of consecutive binary zeros in the encoded bit pattern must be at least as large as a specified non-zero minimum and no greater than a specified maximum. For example, compact discs typically use a code specified as (2,10)-RLL which means that the number of consecutive zeros in the encoded bit pattern must be at least 2 and no greater than 10.
A fourth typical constraint on the encoded binary signal is a requirement for a limit on the low frequency content of the read signal. In many read channel detection systems (for example, a differential phase detection system), a transition is indicated when the read signal crosses a fixed threshold (the threshold between a mark and a space). Any low frequency content in the read signal can cause an offset, restricting the dynamic range of the detection system. In addition track following and focusing signals (collectively referred to as xe2x80x9ctrackingxe2x80x9d signals) are often implemented using the low frequency modulation content of the read signal. Any low frequency content in the read signal due to data patterns may interfere with tracking.
Again in the NRZI format, one state of a signal (for example, the pit, or the mark) is assigned the value +1 and the opposite state (for example, the space) is assigned the value xe2x88x921. A sum of these values is called Digital Sum Variance (DSV) or alternatively Running Digital Sum (RDS). For many detectors, there is a specified maximum DSV or RDS, and any DSV exceeding the specified maximum is likely to cause data read errors, servo problems, or loss of tracking.
In FIG. 1, the process 20 of creating an original disc 14 from original data 12 is illustrated. First, the original data 12 are read as illustrated by step 22. Then, error correction code (ECC) is added to the data. Step 24. The ECC is added to correct errors due to manufacturing defects and reading errors. This is well known in the art.
Next, the data, including the ECC, are encoded to channels bits. Step 26. The encoding step produces a sequence of bits that, collectively, meet the second, the third, and the fourth constraints discussed herein above. Finally, the channel bits, representing the encoded data plus the ECC, are written on an original disc 14. Step 28. In this document, the words xe2x80x9cwritexe2x80x9d and xe2x80x9cwritingxe2x80x9d of a disc includes, without limitation, various technologies and creation techniques to produce, for example, an optical CD or a DVD including stamping, burning, and fabricating.
The process 30 of duplicating the original disc 14 to a duplicate disc 18 is also illustrated in FIG. 1. To copy the original disc 14, first, the above-described steps are applied in a reverse order to retrieve the data illustrated as recovered data 16 in FIG. 1. Then, the above-described steps are repeated, using the recovered data 16 as input, to write a duplicate disc 18.
In more particular, to produce the duplicate disc 18, the original disc 14 is read to retrieve the channel bits. Step 32. Then, the channel bits are decoded to the underlying data plus the ECC. Step 34. Next, the ECC is removed from the decoded data to recover the original data. Step 36. The recovered data 16 need not be saved; however, the recovered data 16 are typically saved, at minimum, on a machine-readable memory such as on random access memory (RAM). Step 38. At this point, the contents of the recovered data 16 are identical to the contents of the original data 12.
To create the duplicate disc 64, the recovered data 16 are read and ECC added for error detection and correction. Steps 42 and 44. The resultant data plus ECC are re-encoded to the channel bits. Step 46. Finally, the channel bits are written on a disc to produce an identical duplicate 18 of the original disc 14.
For recorded digital information, the ability to make an exact copy is often an essential attribute, enabling exchange, distribution and archival of information. Sometimes, however, there is a need to prevent copying. For example, it is illegal to make an unauthorized copy of copyrighted material. Software, music and video providers have a need for distribution of copyrighted works in a digital form while preventing unauthorized copying of those works. There is a need for a method for discouraging copying of digital information.
This need is met by the present invention. According to a first aspect of the present invention, a method of creating a machine-readable medium is disclosed. Copy protection data are encoded for writing onto a disc, the copy protection data, when decoded and re-encoded, producing a tracking prevention sequence of bits.
According to a second aspect of the present invention, an apparatus for writing data onto a disc includes a processor and storage connected to the processor. The storage has instructions for the processor to encode copy protection data for writing onto a disc, the copy protection data, when decoded and re-encoded, producing a tracking prevention sequence of bits.
According to a third aspect of the present invention, a machine-readable medium has instructions for a medium writing machine to encode copy protection data for writing onto a disc, the copy protection data, when decoded and re-encoded, producing a tracking prevention sequence of bits.
According to a fourth aspect of the present invention, a machine-readable medium includes encoded copy protection data, the copy protection data, when decoded and re-encoded, producing a tracking prevention sequence of bits.
According to a fifth aspect of the present invention, a machine-readable medium has a tracking prevention sequence of bits as a result of having re-encoded copy protection data written on the media.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention.